Digital printing systems provide economical, high-speed, high-volume print reproduction. In this type of printing, a continuous web of print media (e.g., paper) or a support mechanism in which the print media is disposed over, is fed past one or more printing subsystems or modules that form images by applying one or more colorants onto the surface of the print media. With a continuous web, various components within the printing system are used to create tension in the web so the web does not shift in the in-track (the direction of movement) and cross-track directions as the web moves through the printing system. The tension is also used to inhibit fluttering (up or down motion) as the web travels through the printing system.
FIG. 1 illustrates a desired position for print media in a printing system. The print media 100 is positioned in a cross track direction to maintain center justification within a media operation zone 102. Typically, the center line 104 of the print media is maintained within acceptable tolerances relative to a device that is performing an operation on the print media while the print media is traveling through (located in) the media operation zone 102. A device that is performing an operation on the print media can be a linehead 106 that jets ink onto the print media or a dryer that dries the ink.
FIG. 2 is a cross-sectional view along line 2-2 in FIG. 1. Each linehead 106 jets streams of ink drops 200 on the print media 100 to produce a print line 300 (FIG. 3). When the center line 104 of the print media is maintained within acceptable tolerances, the print line 300 produced on the print media 100 is straight in the cross-track direction. Additionally, all of the print lines printed on the print media 100 by each linehead 106 are parallel with respect to each other.
FIG. 4 illustrates a top view of web skew in a printing system. Center justification of the print media 100 within the media operation zone 102 is not maintained with web skew. Instead, the print media 100 is skewed in the cross-track direction such that the centerline 104 of the print media is non-linear and curves with respect to the media transport direction of the print media. When the center line 104 of the print media is not maintained within acceptable tolerances, a print line 500 (FIG. 5) printed on the print media 100 by one or more lineheads is not straight in the cross-track direction. Web skew can cause the color planes that are printed on the print media to be misaligned with respect to each other.
Web skew can be caused by one or more factors, including non-linear accuracy of web edge sensors that position the web in the cross track direction, web camber, or misalignment of rollers through the media operation zone 102. Web skew can cause significant delay in the setup of the printing system. In order to make corrections, operators of the printing system must manually evaluate web skew via eye-loop measurements of printed output. The operator must then manually change web servo setpoints to make the necessary corrections to web skew, which is often an iterative process.